A DXF file of a square-to-round transition part in the CAD system shows the flat (unfolded) pattern with the number of segments, or folds, and the 3D wire-frame model. Each is on a separate layer.

A DXF file of a cone transition in a CAD viewer shows the flat pattern, and the 3D model (looking from the top).

Although the shop CNC programmer is just that — a programmer, not a designer — he must constantly take the 2D or 3D data (often nothing but garbage) and somehow turn it into the correct geometry for manufacturing. The geometry is then passed to a CAM system.

The underlying problem for sheet-metal parts, says Bucalo, is that most CAD designers have never actually manufactured such parts and don't know the intricacies of sheet-metal design and manufacturing. For example, many designers would start by making a solid model and then using a Thin Wall Shell command to make what looks like a thin tube. However, the command is not a sheet-metal function and its output cannot be used for manufacturing or unfolding.

Also, he says, many designers are still using 2D software, which doesn't have unfold capabilities. And many designers lack the knowledge to apply real bend factors to parts. They were trained to simply deduct a bend factor, which was calculated years ago by someone long forgotten, and is only correct for 90° bends. It doesn't work for sheet-metal parts.

So to make designers' lives easier, the company provides online calculators for 3D ductwork parts such as cone and square-to-round transitions. Users simply type in the required dimensions. The calculator generates DXF files that include two layers, one with the flat or unfolded pattern, and the other with a 3D wire-frame model of the transition. These are opened in the 2D software, or any program that supports DXF.

The square-to-round transition calculator at sheetmetalguy.com/transition.htmsports a straightforward interface that makes it easy to enter dimensions. Merely type the numbers into the appropriate fields. Then select the number of segments, or folds the transition requires, from a drop-down list (the number must be a multiple of 4, with a minimum of 8 and a maximum of 32), and press Calculate. This brings up a Save window in which to name and save the file. Be sure to save it in the text (TXT) format in your computer.

After saving the TXT file, a popup box reminds users to save it to DXF. This entails using Windows Explorer to browse for the file, and then changing the TXT extension to DXF. After designers import the file into CAD, the software displays another box for selecting the correct units, in this case, Inches.

The cone-transition calculator is also simple to use. Here, users need only type-in the top diameter, bottom diameter, and height to get results. However, the calculator cannot handle complex cones, such as ones with offset or skewed ends. In fact, the site states, "The calculator is based on a true cone segment. The ends are circles, which are parallel to each other. If we receive enough requests for more complex cones, perhaps we will offer a calculator for them as well," says Bucalo.